Local and regional seismic response to injection and production at the Salton Sea geothermal field, southern California

Lia J. Lajoie, & Emily E. Brodsky

Published 2011, SCEC Contribution #1605

California hosts both the largest geothermal resource capacity and highest seismicity rate in the nation. With plans to increase geothermal output, and proven earthquake triggering in the vicinity of geothermal power plants worldwide, it is important to determine the local and regional effects of geothermal power production. This study focuses on relating the volume of fluid extracted from and re-injected into wells at the Salton Sea geothermal field (SSGF) in Southern California to local seismicity rate and increased probability of larger events on nearby faults such as the San Andreas and Imperial faults. Seismic data is obtained from the publicly available Advanced National Seismic System (ANSS) catalog and SSGF injection and production data from the State of California Department of Conservation. We identify triggered earthquakes in the catalog by modeling seismicity in a 15km radius around the SSGF according to an Epidemic-Type Aftershock Sequence (ETAS) method. The model seeks to fit the cumulative seismicity curve from our dataset by optimizing five seismic parameters in accordance with Gutenberg-Richter and Omori's law. The modeled curve is then removed from the dataset to isolate the non-ETAS, or production-triggered, signal. We then formulate a constitutive law to relate the seismicity rate to the driving stress (i.e. volumetric strain in the reservoir). Defining the local stressing rate provides a tool for predicting the effects that production has on regional seismicity rates.
The largest spike in SSGF net production volume over the past 30 years is accompanied by the one of the largest increases in both seismicity rate and moment release within the geothermal field. This indicates a direct coupling between net fluid production volume (volume extracted minus volume re-injected) and seismicity rate and cumulative seismic moment in the field. Three dimensional plots of hypocentral earthquake locations show that seismicity is concentrated on an approximately NW-SE striking plane that dips shallowly to the west. Numerous inactive low angle normal faults with the same orientation have been mapped within and bounding the Salton Trough, suggesting the fault underlying the SSGF could be a reactivated detachment fault. Elevated seismicity rates on the detachment increases the probability of a larger earthquake on the fault that could directly trigger an event on the San Andreas fault.

Citation
Lajoie, L. J., & Brodsky, E. E. (2011). Local and regional seismic response to injection and production at the Salton Sea geothermal field, southern California. Presentation at AGU Fall Meeting 2011.